Avoid warning when loading initial monitor mapping
    
    When started in fullscreen mode with a monitor-mapping configuration
    option, we are getting the following warnings on the terminal:
    
        (process:27428): Gdk-CRITICAL **: gdk_screen_get_n_monitors: assertion 'GDK_IS_SCREEN (screen)' failed
    
        ** (process:27428): WARNING **: Invalid monitor-mapping configuration: monitor #1 for display #1 does not exist
    
    This is apparently because we were processing the fallback monitor
    mapping before the graphic server display was opened, so
    gdk_screen_get_default() returned NULL. This resulted in
    gdk_screen_get_n_monitors() reporting that we had 0 monitors.
    
    This patch moves the fallback monitor mapping parsing from
    virt_viewer_app_init() to virt_viewer_app_on_application_startup(),
    after chaining up to the base class startup() vfunc. The graphic server
    display is opened in the base class vfunc, so by the time that returns,
    we should be able to query the number of monitors.
    
    The patch also adds a check in virt_viewer_parse_monitor_mappings() to
    ensure that we pass a sane value for nmonitors.
    
    Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
    Acked-by: Pavel Grunt <pgrunt@redhat.com>